This specification relates to providing digital content.
The Internet provides access to a wide variety of content items, e.g., video and/or audio files, web pages for particular subjects, news articles, etc. Such access to these content items has likewise enabled opportunities for providing targeted content, such as advertisements. In one type of targeted advertising, the subject matter of a content item accessed by a user can be used to select advertisements, as accessing a content item can be an indication of user interest in the content item's subject matter. For example, if a user accesses a web page discussing automobiles then advertisements related to automobiles are selected and provided to the user.
Users interested in one subject matter category may also be interested in other subject matter categories. Modifying the advertisement candidate set to include advertisements related to other subject matter categories other than the subject matter of the content item the user is viewing and in which the user has an interest or is likely to have an interest can provide a rich set of advertisements that are related to several interests of the user. For example, if it is known that users interested in automobiles are also likely interested in boating, then the advertisements provided to the users can be selected from an advertisement candidate set including both automobile-related advertisements and boating-related advertisements. However, such category similarities are usually inferred from term matching between web pages belonging to particular categories or from data explicitly provided in user profiles. These inferences do not take into account actual user behavior.